1. Field of the Invention
This invention relates to a closure valve for the end closure of a tube thereby enabling the leak-proof trapping of a liquid contained in the tube.
More specifically, the invention relates to the closure valve at the bottom of a spent fuel handling tube used for the refueling of a liquid metal fast breeder reactor.
2. Description of the Prior Art
Continuous operation of a nuclear power reactor results in the eventual depletion of the fuel elements. This necessitates periodic removal and replacement of the spent fuel assemblies. In removing and replacing the spent fuel assemblies, two major problems are encountered. The first is the high radioactivity of the elements. The second is the extreme temperatures generated by the continuing decay of the fuel element fission by-products.
The danger of radioactive exposure or accident is overcome by refueling safeguards which include extensive shielding and precautionary measures such as hermetic seals, remote handling and evacuation and flushing techniques. On the other hand, the excessive heat generated by a spent fuel element requires the removal of spent fuel assemblies to be accomplished with continuous cooling provisions. Two alternatives for handling excessive heat during refueling have been developed in the prior art. When the temperatures involved are low enough, the fuel assemblies are removed directly from the reactor with constant cooling by an air or gas coolant. This method is not acceptable for the fast breeder reactor since the temperatures generated are so high that the rather inefficient gas cooling method would not prevent the fuel element cladding from melting. Accordingly, the high temperatures generated by the continuing decay of a spent fuel assembly of a fast breeder reactor require the second, more efficient, "wet" or liquid cooling method where the spent fuel is continuously immersed and cooled by a liquid coolant.
Methods have been developed in the prior art (for example, see U.S. Pat. No. 3,629,062 issued to Herbert O. Muenchow) which involve the transfer of the spent fuel assemblies from their positions in the reactor core to closed bottom decay pots positioned adjacent to the reactor core within the reactor vessel. Prior art methods of this type require expensive and complicated handling equipment to be located within the reactor vessel. Another prior art method which also requires interior handling equipment provides spent fuel storage space within the reactor vessel. As a result, a larger reactor vessel is needed to accommodate the decaying fuel elements. Any failures that are incurred in the handling equipment on the interior of the reactor are difficult and costly to repair. This results not only from the fact that the interior of the reactor vessel is highly radioactive and is generally inaccessible but also from the fact that the liquid metal of a liquid metal cooled reactor is opaque and in-vessel inspection cannot be easily made. An additional cost is incurred for in-vessel storage by the requirement that a large inventory of fuel assemblies be maintained since the decaying spent fuel assemblies in their decay positions in the interior of the reactor pressure vessel are sealed into the reactor pressure vessel until the next refueling. Periodic refueling is required on the average of once a year while the decay time required to bring the fuel assembly to cooler temperatures is only 120 days.
An alternate solution is possible and involves the direct removal of the spent fuel assembly from the reactor and immediate transportation of this spent fuel assembly to an external decay tank. Such a solution requires the spent fuel handling machine to have provision for continuous wet cooling. The handling machine must also be open at one end for the direct insertion and removal of the spent fuel assembly. The liquid metal coolant of the reactor is used as the coolant of the handling machine and is trapped in the handling machine when the spent fuel assembly is inserted into the machine. Since the liquid sodium of a liquid sodium cooled fast breeder reactor is radioactive and has extreme pyrophoric properties, provisions must be made for a leak-proof handling tube end closure valve.